Skeletal muscle fatigue has been defined as the inability of muscle to elicit a desired force output which it is normally capable of producing. The development of muscle fatigue affects a number of everyday activities including decreased exercise performance and worker productivity and is associated with increased susceptibility to orthopedic injury and precipitation of muscle soreness and damage. At present, the mechanisms which cause skeletal muscle fatigue are not fully understood. [It has recently been established that fatigue is associated with diminished maximal Ca2+ activated force and Ca2+ sensitivity of the contractile apparatus. Unfortunately, it is not clear if these changes are the direct result of fatiguing activity or are secondary to the accumulation of metabolic waste products (e.g. H+, P[i] and ADP). In addition, it is not clear if fatigue and/or its byproducts alter the relationship between force and Ca2+ by affecting the transitions of the cross-bridges to and from the force generating state (cross-bridge cycling kinetics) or by modulating cross-bridge recruitment and force generation. The experiments described in this proposal are designed to determine l) if fatigue directly alters maximal force production and Ca2+ sensitivity of the contractile apparatus and 2) if fatigue- and/or metabolite-induced changes in the relationship between force and free Ca2+ are due to altered cross- bridge cycling kinetics or to altered cross-bridge recruitment and/or their average force generation. This will be accomplished by examining cross-bridge recruitment, force generation and cycling kinetics in skinned skeletal muscle fibers taken from rested and fatigued muscles and exposed to incubation media that is formulated to mimic "rested" and "fatigued" intracellular environments. These parameters will be examined using measurements of the rate constant of force redevelopment after a period of isotonic shortening, complemented by measurements of isometric force, ATPase activity and stiffness during isometric contraction at various levels of free Ca2+. In general, the results of the proposed experiments will provide much needed information regarding the mechanisms of skeletal muscle fatigue. These findings should also lead to a better understanding of the fatigue process and provide an experimental basis for developing means of alleviating and avoiding muscle fatigue.]